“Leitungs-Differentialschutzeinrichtung PQ 741” [Line differential protective device PQ 741], FIG. 7, page 11 from the company Alstom (retrievable under “datasheet (6)” in German, file name “PQ741—06—05511GR.pdf” at “http://www.tde.alstom.com/pc/bases/gr/products.nsf/e47708d69e62a70480256816003800f3/372f6914871f304bc1256841002cdaa1?OpenDocument”) discloses a first digital protective device arranged at the end of a first section of an electrical power transmission line. At the other end of the section of the electrical power supply line, a second digital protective device is provided. The two protective devices form differential protection for monitoring the section for a fault. To perform the differential protection function, the two protective devices are connected to one another for the purpose of data transmission. To this end, the first digital protective device is connected to one end of a two-pair electrical data transmission linevia an interconnect device. The other end of the two-pair electrical data transmission line has the second digital protective device connected to it via a second interconnect device. Data are transmitted between the two interconnect devices on the basis of a special data transmission protocol. In this context, the two interconnect devices are designed specifically for digital data transmission between the two protective devices via the electrical data transmission line. This involves transmitting digital data from the first interconnect device in the direction of the second interconnect device using one pair of the electrical data transmission line. For digital data transmission from the second interconnect device to the first interconnect device, the other pair of the electrical data transmission line is provided. The respective specific design of the interconnect devices allows data transmission between the two interconnect devices on the two-pair, electrical data transmission line over a data transmission line length of up to 15 km.